1. Field of the Invention
The present invention relates to a method of manufacturing a circuit component such as a stator for a variable resistor, which comprises a film electrical element such as a resistor film, a conductor film or the like, for example, provided on the surface of a substrate. More particularly, it relates to a method of forming the film electrical element on the surface of the substrate.
2. Description of the Prior Art
A film electrical element such as a resistor film or a conductor film is provided on the surface of a substrate of resin, for example, most typically by screen printing. Namely, paste prepared for forming a resistor or conductor film is applied onto the surface of a substrate by screen printing, thereby to provide a paste film of a desired pattern, which is then dried and thereafter fired. Thus, obtained is a circuit component which comprises a resistor film and/or a conductor film of desired pattern(s) provided on the surface of a substrate.
In the aforementioned method, however, it has been difficult to provide the film electrical element on a non-planar surface such as a curved surface, for example. Further, thickness of the film electrical element is directly influenced by the surface state of the substrate. Therefore, if the substrate itself is cambered or its surface is uneven, the film electrical element cannot be provided in uniform thickness on the surface of such a substrate. Thus, it is difficult to regularly stably obtain film electrical elements having desired electrical properties. This leads to dispersion in electrical property between circuit components thus obtained.
A typical example of a circuit component is a stator for a variable resistor, which stator generally comprises an electrical insulating substrate and a resistor film provided in a C-shaped configuration, for example, on its surface. On the other hand, the variable resistor comprises, as rotor structure, a brush which is brought into contact with the resistor film to slide along the longitudinal direction of the resistor film. This brush is driven to rotate so that the angle of its rotation is substantially proportionate to a resistance value provided by the variable resistor, in general. However, it is difficult to attain linear relation between the rotational angle of the brush and the resistance value due to the ununiform thickness of the resistor film resulting from the aforementioned cause. Particularly in a small variable resistor such as a chip-type one, the relation between the rotational angle and the resistance value is significantly influenced by the aforementioned ununiform thickness of the resistor film. Further, a resistor film provided by screen printing tends to have an inverted U-shaped surface in cross section. When a brush slides along the surface having an inverted U-shaped section, a contact point of the brush to the resistor film may deviate in the cross direction of the resistor film with sliding of the brush. This also inhibits linear relation between the rotational angle of the brush and the resistance value. In the small variable resistor such as a chip-type one, further, the resistor film cannot be strongly adhered to the substrate because of its small area, and hence the resistor film may be partially separated from the substrate.
Japanese Patent Publication Gazette No. 11125/1982 suggests means for solving the aforementioned problems of ununiform thickness of the resistor film, inhibition of linear relation between the rotational angle of the brush and the resistance value caused by the ununiform thickness and low adhesion between the resistor film and the substrate. This gazette discloses a method of applying resistive paste on a metal plate having a smooth surface to form a paste film, laminating a plurality of prepreg sheets respectively impregnated with thermosetting resin materials having specific properties and heating the laminate with pressurization for hardening the multilayer of prepreg sheets to provide a substrate and separating the metal plate from the substrate to leave a resistor film obtained from the resistive paste on the substrate.
According to the aforementioned technique, the resistor film can be easily obtained in relatively uniform thickness due to smoothness of the surface of the metal plate, while adhesion of the resistor film to the substrate can be improved since the resistor film is embedded in the substrate to define a surface which is even level with that of the substrate. However, this technique is restricted to a flat substrate, and inadequate to manufacture a substrate having a complicated configuration. For example, it is difficult to manufacture a stator having a relatively complicated configuration such as that for a variable resistor provided with a resistor film and having a function for holding lead terminals, unless further steps are introduced.
Further, electrical properties of a circuit component comprising a film electrical element cannot be checked until the film electrical element is actually provided. This is because the composition of resistive or conductive paste for forming film electrical elements is slightly varied with manufacturing lots while electrical properties of finished film electrical elements are influenced by various conditions such as printing, drying, baking and the like. In actual manufacturing, therefore, a small number of prototypes are first prepared for each lot of the resistive or conductive paste for checking electrical properties of the prototypes, to thereafter start mass production. This process is adapted to prevent total defectiveness of products included in any manufacturing lot which cannot attain target electrical properties. In general, however, two to four hours are required for preparing such prototypes and checking electrical properties thereof, while stopping the production facility. Thus, productivity is reduced every time the lot is changed, leading to increase in production cost.